However, what is worth noting is that this configuration for Papal Masses is actually quite new as far as Masses are concerned. Generally speaking, prior to the 1970's, the only time one would see the papal throne placed before the altar of St. Peter's was for occasions in which it was not a Papal Mass. Here is one example in which the pontiff is seated in this same general position, while down before the confessio a temporary altar has been setup for the celebration of the Byzantine Liturgy during the conciliar period.
In order to accomplish these types of arrangements, it was necessary to cover over approximately one third of the confessio before the high altar. For those not familiar, the confessio is the space opened before and beneath the high altar in which are generally deposited beneath the relics of a martyr or what not. In ther case of St. Peter's of course, that is none other than St. Peter himself.
Since the 1970's this ad hoc arrangement has become effectively permanent -- by which I mean the temporary platform is basically never removed. Such is the seeming permanence of this arrangement that people might be confused of which I speak. To illustrate, the following photo shows the moveable platform (left in colour):
Here is another view of it, taken from another angle with the front steps removed: